The present invention relates to a storage type data communication terminal which provides communication services between terminals and between terminals and a center according to a communication sequence constituted by call connection, communication, and call release phases.
For example, as disclosed in Japanese Patent Laid-Open No. 61-156095, a conventional power control scheme is used to prolong the service life of the LCD section of a display connected to an apparatus, and realize labor saving and power saving in the apparatus itself. The above prior-art invention can be applied to communication terminals. For example, the power supply of a communication terminal may be controlled depending on the presence/absence of data in the buffer in the communication terminal. More specifically, the conventional technique may be applied in such a manner that the power supply of the apparatus is kept on when data is present in the buffer in the apparatus.
If, however, the above conventional technique is applied, the following problems are posed. The reasons why the problems are posed will be described with reference to sequences in FIGS. 5 to 7. FIG. 5 shows a communication sequence to be followed when communication is to be performed between storage type radio data communication terminals (to be referred to as TEs hereinafter). FIG. 6 shows a communication phase in the communication sequence in FIG. 5 in detail. FIG. 7 is a communication sequence to be followed when the power supply of a TE is reset by an operator in the sequence in FIG. 5. In this case, the term "TE" means a radio data communication terminal having a function of automatically dividing data transmitted from an external information processing device (to be referred to as a DTE hereinafter) on the operator side in a given unit (to be referred to as a packet hereinafter), and checking an error in units of packets in radio communication with a radio data communication terminal.
The following case is posed as the first problem.
Assume that a TE is operated according to the sequence in FIG. 5. In the connection phase shown in FIG. 5, upon reception of a connection request message from the TDE of a terminal T1, the TE of the terminal T1 transmits this connection request message to the TE of a terminal T2 through a radio channel. The TE of the terminal T2 receives the connection request message from the TE of the terminal T1 and outputs a termination designation message to the DTE of the terminal T2. The TE of the terminal T2 also returns a connection response message to the DTE of the terminal T1 through the TE of the terminal T1. Thereafter, in the communication phase, transmission data is exchanged between the DTE of the terminal T1 and the DTE of the terminal T2 through the TEs of both the terminals. When exchange of transmission data is completed, the TE of the terminal T1 transmits a disconnection request message from the DTE of the terminal T1 to the TE of the terminal T2 through a radio channel in the disconnection phase. The TE of the terminal T2 receives the disconnection request message from the TE of the terminal T1 and outputs a disconnection designation message to the DTE of the terminal T2. The TE of the terminal T2 also returns a disconnection response message to the DTE of the terminal T1 through the TE of the terminal T1.
FIG. 6 shows the communication phase in FIG. 5 in detail. Upon reception of transmission data from the DTE of the terminal T1, the TE of the terminal T1 divides the data into 1st to Nth packets and transmits them to the TE of the terminal T2 through a radio channel. Every time the TE of the terminal T2 receives a packet from the TE of the terminal T1, the TE returns an acknowledgement message to the TE of the terminal T1. Upon reception of all the packets, the TE of the terminal T2 outputs the transmission data to the DTE of the terminal T2. When the above prior-art invention is applied to this terminal, the power supply of the apparatus may be turned off upon completion of a predetermined communication sequence.
As is well known, the electric field level of a radio channel greatly varies, and the channel quality is not always good all the time. For this reason, when the channel quality deteriorates, the TE which operates according to the sequence in FIG. 5 repeatedly transmits a packet or an acknowledgement message with respect to a packet in the communication phase (FIG. 6). Since this operation is performed by the TE itself, the operator of the TE and the DTE cannot grasp the operation state at all. In this case, the operator may forcibly turn off the power supply of the apparatus by operating the power switch, thereby forcibly disconnecting the channel.
If, however, the conventional technique is applied, since supply of power is automatically maintained until a predetermined sequence is completed, the operator cannot forcibly disconnect the channel through which communication is currently performed. For this reason, if the above prior-art invention is applied, in a state wherein a communication channel is unstable, communication cannot be ended until the timeout of the communication sequence.
In applying the above prior-art invention to a communication terminal, even when channel quality deteriorates, the operator cannot immediately disconnect the channel, but needs to wait until the timeout of the communication sequence. This causes an inconvenience to the user.
The following case is posed as the second problem. Assume that the TE operates according to the sequence shown in FIG. 5, and the above prior-art invention is not used. When the operator turns off the power supply of the TE of the terminal T1 while the TE of the terminal T1 is transmitting data as shown in FIG. 7, the TE of the terminal T1 cannot receive an acknowledgement message from the TE of the terminal T2 with respect to a packet transmitted immediately before the power supply is turned off. In addition, since the sequence of packet transmission and acknowledgement reception is directly performed between the TEs, the operator cannot perform transmission/reception control. For this reason, the operator cannot know up to which packet transmission has been completed. In the case shown in FIG. 7, since the TE of the terminal T1 cannot receive an acknowledgement message with respect to the second packet, the Te cannot check whether the terminal T2 could transmit the second packet.
If, therefore, the operator turns off the power supply of the TE while the TE is performing communication, the operator cannot know whether data transmitted immediately before the power supply is turned off is received or not. In order to reliably transmit the data to the destination, the operator must transmit the same data as that transmitted immediately before the turn-off operation of the power supply again. The transmission efficiency deteriorates from the viewpoint of the operator.